The invention relates to the field of workpiece gauging systems, and more particularly to a non-contact gauging system of this type.
In industrial assembly operations it is desirable to know the position of a surface to decide, for example, if the workpiece size meets specifications within acceptable tolerances. Such a gauging operation must be done quickly and accurately in an environment where it is often undesirable to make physical contact with the workpiece, especially where the workpiece has rough surfaces and/or rough edges which reduce the precision and repeatability of any measurements made by physically contacting the workpiece. In addition, it is desirable to do such gauging by automated apparatus to increase the gauging speed and minimize errors as compared with manual gauging.
Various techniques have been proposed for performing non-contact measurement of parts in an industrial environment. One example is referred to in Waters, J. P., "Gauging by Remote Image Tracking," Optical Engineering, Volume 18, No. 5, pages 473-477, September-October 1979, and involves an optical triangulation technique. As proposed therein, a spot of light is projected from a gauge head onto the part being gauged. An image of this spot is focused on the center of a tracking photodetector. If the part is moved in a direction perpendicular to the projection axis, the focused spot on the surface of the photodetector will move as the surface curvature of the part changes. Various signals are derived from the photodetector output and used to drive the system to a system null. Another gauging system is proposed in U.S. Pat. No. 3,994,589. The patent refers to measuring the portion of an illuminated spot on a workpiece surface which is contained within a predetermined aperture and using this as an indication of the position of the illuminated surface. While such proposals may be useful in some environments, a need remains for an accurate, fast, reliable and relatively inexpensive system for finding the position of a workpiece relative to a reference. The instant invention is directed to meeting that need.
According to the invention, a method of gauging a workpiece surface and a system for carrying out the method are disclosed, in which an arbitrary reference is established by placing a surface at one or more known positions, illuminating a spot on that surface with a radiant energy source, remotely detecting the illumination of that surface with an illumination measuring device and producing, in response to the detected illumination, a reference signal which is, in effect, a measure of the system response to the known position or positions of the surface. To find the unknown position of a workpiece surface relative to the arbitrarily established reference, the workpiece surface is illuminated with the radiant energy beam and the illumination measuring device detects the illumination of the workpiece surface to produce signals which are converted to a test signal. The test signal emphasizes the desirable components of the illumination measured with the measuring device and de-emphasizes the undesirable components of the measurement. This test signal is combined with the reference signal to produce a measurement signal indicative of the position of the workpiece surface relative to the reference. This measurement signal is used in various ways as a representation of the sought position of the workpiece surface. The geometric relationship between the radiant energy beam and the workpiece surface is such that the illuminated spot moves along the workpiece surface with movement of the workpiece surface relative to the reference along a measurement direction.
The radiant energy source may be a source of a light beam, either coherent or non-coherent, or a source of electro-magnetic energy in a spectrum outside the visible one. The illumination measuring device may be, for example, a solid state array scanner, or an Image Dissector or a Vidicon device, or another similar array-type imaging device which is responsive to the particular type of radiation of the radiant energy source.
The illumination measuring device detects the illumination at each of a number of small areas on the workpiece surface to produce respective initial signals related to the illumination levels of the respective small areas of the workpiece surface. These initial signals are converted to a test signal which is a function of the difference in overall illumination between selected larger areas of the workpiece surface. It is this test signal which is combined with the reference signal to produce the measurement signal indicative of the sought position of the workpiece relative to the reference.
As discussed in detail below, other important aspects of the invention relate to the types of signals which are generated at various stages of the gauging process, to the way these signals are combined with each other and with other signals, and to the way the final measurement signal is produced and utilized, these aspects of the invention contributing to making the system less sensitive to extraneous, undesirable influences and more responsive to what is in fact sought: the position of the workpiece surface relative to the reference along a measurement direction.